I agree. The cable is sufficient to carry power over a long distance (as long as you use thick enough wire), but the issue is in the signal quality, especially if the mcu is sending out serial clock at a high speed. You can feel free to give it a try, but I suspect that it won’t be very reliable. Instead of wiring the expansion board 100 feet away, is it possible to place the expansion board close to the main controller, and extend the wires to those valves?